Conventionally, a gear mechanism is widely used as a driving mechanism for an image forming unit such as a photoconductive drum and a transfer roller in an image forming apparatus such as a copier. A gear mechanism composed of resin gears is inexpensive. However, this gear mechanism tends to cause banding and jitter in an output image due to reduced rigidity, gear mesh vibration and the like. Thus, there is also a method using a traction drive with high rigidity and no gear mesh as a driving mechanism. However, this method is very high in cost and unsuitable for the development of cost competitive products.
Accordingly, a metal belt deceleration driving method for mounting and driving a metal belt between pulleys on two shafts has been and is being studied. In this method, a pair of pulleys (drive pulley and driven pulley) having different diameters are used to realize a predetermined speed reducing ratio. The metal belt mounted on these pulleys is shaped to be linear between the pulleys and curved along the pulleys. Thus, conventional metal belts have a problem that metal fatigue occurs due to the repetition of such a shape change and durability against stresses (tensile stress, bending stress) applied to the metal belts from the pulleys is reduced to cause fracture. To solve this problem, a method has been proposed which gives durability against tensile stress by applying a solution treatment to a metal belt and, thereafter, plastically deforming the metal belt and applying an aging treatment to actively remove a remaining stress (Japanese Examined Patent Publication No. H05-33284).